The present invention relates to a method and a device for measuring in situ the gap separating two given elements in a tubular pipe.
A flexible pipe usually includes an end fitting at each end. The end fittings are members which allow the flexible pipes to be connected to stationary structures such as a structure floating on the surface of the sea and anchored to the seabed or else a structure placed on the seabed such as, for example, a well head or a manifold. End fittings may also be used for joining lengths of flexible pipe when the latter has a long length. The greater the length of the pipe (several kilometers), the larger the number of mid-connectors used.
The pipes targeted by the present invention are those which are called xe2x80x9crough borexe2x80x9d, which comprise a carcass as the innermost element and placed over this carcass is an inner sheath, of the type described in the American Petroleum Institute 17J and American Petroleum Institute 17B standards.
In general, one end of the pipe is mounted and fastened inside the end fitting by suitable means which are well known and will not therefore be described.
An end fitting has two functions, one external and the other internal. The external function consists in connecting the end fitting to a structure regarded as being stationary, using a connection flange for example. The internal function is provided by a vault and clamping means which allow the sealing continuity of the flexible pipe to be maintained, the inner sheath being clamped onto the carcass by means of, for example, a monocone. The end of a pipe is mounted and fastened in the end fitting in such a way that a stop ring, welded to the end of the carcass, is located at a certain distance from the vault of the end fitting.
However, during any period of use of the pipe, it is necessary to measure the gap separating the stop ring from the vault of the end fitting. In fact, even a very slight displacement of the stop ring, integral with the carcass, with respect to the vault of the end fitting may mean a relative displacement of the inner sheath with respect to the pressure vault. Such a displacement of the inner sheath may cause a loss of sealing, which would be prejudicial to proper operation of the pipe. It would then be essential to change the pipe. The relative displacement of the inner sheath with respect to the pressure vault may arise from a reduction in the clamping force exerted by the monocone on the inner sheath. Modification of the properties of the inner sheath may also cause a displacement of the sheath, the modification possibly being a result of temperature fluctuations of the fluid flowing in the pipe.
In non-destructive testing of a pipe, such as the testing described for example in Patent FR-B-95/08635, it is possible to use an eddy-current sensor to detect defects in the product to be tested. Eddy-current testing consists in mounting a sensor on a pig which is moved in the pipe using suitable means, such as a pressurized fluid. The sensor produces eddy currents in the pipe which generate in turn a variable reactive magnetic field. The field affects the impedance of a receiving coil placed near the surface to be tested. In the presence of a defect, the circulation of eddy currents is disturbed by the defect, thereby causing variations in the reactive field and therefore in the impedance of the receiving coil. Measuring the impedance should make it possible to detect the presence of defects in the product. The precise position of the defects is not known but can be determined to within about one meter. However, the amplitude of the signal coming from the receiving coil depends on the dimensions of the defect, mainly the depth and the width of the defect. The amplitude of the signal also depends on the electromagnetic properties of the walls of the defect.
However, the eddy-current sensor as normally used cannot allow the size of the defect to be easily determined. This size can be measured only if the speed of movement of the sensor, on passing over the defect, is known very precisely, and only if the speed is constant. One solution recommended by specialists consists in bringing in and immobilizing a scanner right at the defect to be measured and in scanning the defect at a predetermined and constant speed. Such a solution is difficult to employ in the case of a long pipe having a large number of defects to be measured, since it in particular requires the defects to be accurately located along the pipe in order to bring the scanner in line with the defect. The scanning may be carried out by eddy currents, ultrasound or other means.
However, scanning is limited to lengths of flexible pipe not exceeding 5000 m because of the capstan effect which is liable to occur in the curved parts of the flexible pipe. The capstan effect possibly in turn induces an elongation of the umbilical at the end of which the sensor is mounted.
In order to avoid the capstan effects, it is conceivable to use autonomous pigs carrying the sensors and the data acquisition and storage means. The sensors mounted on these autonomous pigs may either be of the leakage-flux type. But then there is a difficult problem to be solved when the strip and/or the ring is made of non-magnetic material. The sensors may be of the ultrasound type, and then any impurities located in the defect may falsify the measurement appreciably since the defect may be hidden by these impurities, and the defect is then no longer detected.
A coding wheel cannot be envisaged for bringing the scanner onto the defect since not only can it slide on the carcass but it is also disturbed by the irregular shape of the internal wall of the carcass.
Another solution would consist in calibrating the current sensor according to a given type of defect. To do this, the sensor would have to be calibrated using several pipes which serve only for the calibration and which must have very precisely the defect desired, both in its size and its structure and the constituent materials of the flexible pipe and of the end fitting in which it is desired to detect a defect. These calibration pipes are new every time a pipe to be tested is changed. This entails a considerable cost and a very lengthy measurement time, including the calibration time. In reality, such a solution cannot be envisaged on an industrial scale.
It is also possible to use a so-called low-resolution sensor and to carry out a correlation between, on the one hand, the shape and amplitude of the signal and, on the other hand, the size of the defect. But this requires tedious and stringent calibration techniques.
The amplitude of the processed signal increases with the width of the defect detected.
Another technique consists in using a so-called high-resolution sensor which allows the edges of the defect to be detected sequentially. In this case, the amplitude of the processed signal remains constant whatever the width of the defect, but this would not avoid a calibration as indicated above.
The object of the present invention is to remedy the aforementioned drawbacks of the known solutions and to propose a method and a device for accurately measuring the gap separating the vault of the end fitting from the stop ring or carcass, which are not sensitive to the speed of movement and/or variations in the speed of movement of the pig, which does not require knowing in advance the position of the gap or gaps when the pipe comprises several end fittings along its entire length, which are compatible with the measurement environment and which minimize the complexity of the electronics for data processing and storage.
It has been found that, contrary to what was accepted by the experts, the eddy currents could be used simply by combining at least two sensors mounted in tandem and separated by a fixed interval, as will be described below.
The subject of the present invention is a method for measuring the gap separating a vault of an end fitting from the end of an inner carcass of a pipe, of the comprising in:
moving autonomous measurement means inside the pipe at a speed which may vary,
carrying out at least one series of measurements during the movement of the measurement means in the end fitting,
storing the measurements in the form of signals in at least one memory,
processing the said signals,
and it further comprises
detecting, in real time, the passage of the consecutive transitions between the vault of the end fitting and the end of the terminal part of the pipe,
triggering the measurements for a predetermined time between the detections of the said transitions and
processing the stored signals in order to determine the gap separating two consecutive transitions.
Another subject of the present invention is a device for implementing the above method. The device comprising a support which is propelled inside the pipe by means such as a pressurized fluid, at least one high-resolution eddy-current sensor mounted on the support, means for storing the signals delivered by the sensor, at least two eddy-current sensors which detect in real time the transitions located on either side of a gap delimited between the vault of the end fitting and the stop ring of the carcass, and the said sensors are located a short distance from the internal wall of the pipe.